The banks include five global financial institutions based in the United States: Bank of America, Citigroup, Goldman Sachs, Morgan Stanley and JPMorgan Chase.
In a statement Citigroup called the ratings cut "arbitrary."
Banks are also countering the downgrades by letting investors know they are better capitalized than they were before the 2008 financial crisis, The Wall Street Journal reported Friday.
Foreign banks hit with downgrades include Deutsche Bank AG, Barclays, HSBC Holdings, Credit Suisse Group AG, Royal Bank of Scotland Group PLC, BNP Paribas, Credit Agricole SA and Societe Generale.
Credit ratings at UBS in Switzerland and the Royal Bank of Canada were also downgraded, the Journal said.
On Wall Street, stock markets rose with help from financial firms.
In midmorning trading, shares of Bank of America rose 0.03 percent, while shares at JPMorgan Chase rose 0.74 percent.
Markets were partly on a rebound after sharp losses Thursday. But investors had already largely priced in the downgrades, as Moody's announced early in the year it was reviewing 100 banks.
The downgrades will make it more expensive for the banks to borrow and could make it harder for them to raise more capital.